Wheel / speedo re-calibration

[kcphoto21]

Hey guys,

First of all sorry if this is not the correct thread to ask this question.

I have a 2016 Mustang that came with stock 20" wheels. Problem is the tires are summer only. I live in midwest where it snows. So, I bought stock 18" wheels and tires from eBay thinking I will run all seasons in winter and summer only wheels in summer.

Wanted to ask if I need to re-calibrate the speedometer. Here are the dimensions.

Stock summer only: 265/35R20 = (265 * 0.35)*2 + (20"*25.4) = 27.30 inches
All Season tires: 235/50R18 = (235 * 0.5)*2 + (18"*25.4) = 27.25 inches

There is a very small change in diameter, do I still need to re-calibrate the speedometer.

Many thanks!

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[dgc333]

I'm assuming the car takes the speed reading from the front wheels but can anyone confirm this? My fronts are overall shorter then the rears so I just want to know which wheels to adjust my speed by.

I think it goes without saying that the speedo runs off the front wheel (think lack of wheel spin), so that is the tire that needs to closely match the OEM diameter.

www.wheel-size.com

I have not seen a RWD vehicle that gets its speed info from the front wheels since the 62 VW Bug I had in High School.

The speedo sensor is on the output shaft of the transmission. So RWD it's the back wheels, FWD it's the front wheeks.

 

[dgc333]

Speedometer is measured by the rear wheels, track apps are measured by the front. Different tire height changes the speedometer error by a certain percentage, not a flat rate number. My PP Speedo reads about 4% low. Meaning I am actually getting 4% better gas mileage than indicated, and also a slight extended warranty lol...Not that I'll be using it to it's extent. I will probably tune and mod by 40k.

Track Apps has NO input from either front or rear wheels. It uses an accelerometer and timer to calculate distance and speed as well as acceleration.

Same as the track apps that you can put on your smart phone.

Also if your speedo reads low, that is the odometer, then you are not recording as many miles as you have actually traveled and your calculated gas mileage will be lower.

If the speedometer is reading low but the odometer is correct then there is ZERO impact to your calculated mileage.

 

[gsxr1300]

you can use Whipple's new tool to recalibrate for tire size changes.

 

[RubyRed15]

Hey guys,

First of all sorry if this is not the correct thread to ask this question.

I have a 2016 Mustang that came with stock 20" wheels. Problem is the tires are summer only. I live in midwest where it snows. So, I bought stock 18" wheels and tires from eBay thinking I will run all seasons in winter and summer only wheels in summer.

Wanted to ask if I need to re-calibrate the speedometer. Here are the dimensions.

Stock summer only: 265/35R20 = (265 * 0.35)*2 + (20"*25.4) = 27.30 inches
All Season tires: 235/50R18 = (235 * 0.5)*2 + (18"*25.4) = 27.25 inches

There is a very small change in diameter, do I still need to re-calibrate the speedometer.

Many thanks!

You're good. You'll see more diameter change from tread wear than 0.05".

 

[Mike Jesse]

I have the 235-18's on my Mustang, soon to change out to a 315/35/20.
This represents a 5% change.
i was under the impression that I could change this value using the SCT 4 tuner.

 

[DivineStrike]

Odometer is based on wheel speed which is calculated by mph. My distance traveled is off by just as much as my speedo is. I see that is what you were saying I think I only skimmed some of your post

As for the track apps you comment makes no sense. It is calculated based on speed which is measured by the wheels. It will stop going up if you don't increase your speed. If I remember correctly my speedo does hit 60 in 2nd. I'd have to double check though

A previous test showed engineering mode only hitting 59. The speedo looked like it was 60. I believe my gps showed 60. That I will have to actually test lol I just referred to an old post of mine

 

[kcphoto21]

Thanks RubyRed15 and dgc333.

 

[dgc333]

Odometer is based on wheel speed which is calculated by mph. My distance traveled is off by just as much as my speedo is. I see that is what you were saying I think I only skimmed some of your post

As for the track apps you comment makes no sense. It is calculated based on speed which is measured by the wheels. It will stop going up if you don't increase your speed. If I remember correctly my speedo does hit 60 in 2nd. I'd have to double check though

A previous test showed engineering mode only hitting 59. The speedo looked like it was 60. I believe my gps showed 60. That I will have to actually test lol I just referred to an old post of mine

Odometer has nothing to do with speed. It is strickly a distance traveled measurement based on revolutions of the tires, whether you are going 10 mph or 100 mph the odometer will read the same for the same distance. The speedometer measures the rate at which you cover distance. Errors in the odometer measurement will impact the speedometer but errors in the speedometer do not impact the odometer. Errors come into play as the tires wear down (you don't go as far per revolution), lots of burn outs add distance the car has not actually covered, you install tires that are larger or smaller than stock or you change the rear end ratio (speedo signal comes from a sensor on the transmission output).

If you go to the tire sites like Tire Rack and look at the specs for a given size there will be a spec of how many revolutions of the tire it takes for the tire to travel a mile. Taller tires less revolutions, shorter tires more. It's this revolution per mile number you change to compensate for different size tires. The SCT X4 tuner allows you to adjust the number in increments of 5.

They could use the speedometer signal for the track app but they don't because it would be a very inaccurate method when you consider wheel spin. Depending on what rear end you have you can easily exceed 60 mph in second gear and these cars have enough power to spin the tires through second gear. You could barely be moving and the track app could be recording a 0-60 time. That is the reason they use an accelerometer it has no connection to the powertrain and measures the acceleration of the car. Integrate the acceleration over time and you have velocity, integrate velocity over time and you have distance.

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